High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
Such polyesters have not been widely accepted for use as molding resins, however, until only fairly recently, because of their relative brittleness in thick sections when crystallized from the melt. This problem was overcome by varying the crystal texture, e.g., by using two step molding cycles or including nucleating agents, and by molecular weight control. This permitted the marketing of injection moldable poly(ethylene terephthalates) which typically, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, and lower surface friction. High homologs of poly(ethylene terephthalate), e.g., poly(1,3-propylene terephthalate) and poly-(1,4-butylene terephthalate) crystallize more rapidly from the melt and may, therefore, be used without two step cycles or nucleating agents, directly in molding compositions.
However, although the thermoplastic polyesters are very useful materials, their application has been severly hindered by the fact that they burn readily and are extremely difficult to render fire retardant.
It has been proposed to incorporate conventional flame retardant compounds, e.g., halogenated or phosphorus-containing organic compounds, with or without synergists, e.g., antimony compounds, non-conventional compounds, e.g., tetrabromophthalic anhydride, to render polyester compositions flame retardant, but these are not generally satisfactory to meet the more rigid requirements, e.g., those of the Underwriters' Laboratories specifications. Moreover, some degradation of the polyester component is seen; the burning material drips and can ignite combustible materials beneath it; and after-glow remains as a substantial problem.
Three main factors apparently are responsible for the unusual difficulty in rendering polyester compositions fire retardant-- in comparison with other thermoplastics, for example. These factors are:
(a) conventional flame retardant additives, e.g., halogenated compounds, phosphorus compounds and antimony compounds, do not appear to be very effective when used with polyesters because the polyesters contain a relatively high oxygen content;
(b) polyesters have a tendency to drip while burning, and it is difficult to prevent the dripping; and
(c) polyesters are subject to serious degradation in the presence of a number of conventionally used flame retardants, with a loss in physical properties.
After evaluating many materials, it has now been found that certain phosphorus compounds are very specifically efficient in providing flame retardancy. Many of them are active at very low phosphorus levels, e.g., 1% or less by weight. Surprisingly, other, quite similar, phosphorus compounds have little or no effect on normal flammability of polyesters.
It has been discovered, for example, that brominated aryl phosphates, e.g., tris(4-bromophenyl)phosphate, are very active flame retardants for poly(alkylene iso- and terephthalates) at concentrations of, e.g., 10 parts by weight per hundred parts by weight of polyester, providing a rating of SE-O in the very stringent Underwriters' Laboratories UL-94 test. Surprisingly, these results are obtained in the absence of synergists, with only 0.5% by weight of phosphorus, and with only 3.9% by weight of bromine in the composition. Unexpectedly, the corresponding chlorinated aryl phosphates, e.g., tris(4-chlorophenyl)phosphate, have a very low flame retardant activity, showing the surprising specificity of bromine in one aspect of the present compositions.
It has also been discovered that partially aromatic phosphonates, e.g., poly(1,4-cyclohexylene dimethylene)phenyl phosphonate, poly(1,4-phenylene)phenyl phosphonate and poly(4,4'-isopropylidene diphenylene)phenyl phosphonate, are very active flame retardants for poly(alkylene iso- and terephthalates) at concentrations of, e.g., 10 parts by weight per hundred parts by weight, providing an Oxygen Index of 29- 30% in the test according to ASTM-D 2863. Surprisingly, these results are obtained in the absence of halogen or synergists, and with only 0.8- 1.2% by weight of phosphorus in the composition. Unexpectedly, the corresponding, almost identical, aromatic phosphates, give an Oxygen Index of only about 24.0% maximum which is only slightly improved over that of the normally flammable polyester resin itself.
As further illustrations of the highly specific activity of the flame retardant additives discovered to be useful, it has been found that aliphatic phosphonates are much less effective; di(arylalkyl)arylalkyl phosphonates which do not contain an aryl-phosphorus bond are efficient flame retardants, but are unstable and degrade the polyester; hydrogen phosphonates are much less effective as flame retardant agents and degrade the polyester; and alkylphenylphosphinates, which contain an alkyl-phosphorus bond-- not interrupted by oxygen-- are unstable and degrade the polyester. Trivalent phosphorus compounds, including phosphites, phosphonites and phosphines are also much less effective than the present pentavalent phosphorus containing compounds.
It has been found that any tendency to drip, while burning, can be overcome by adding to the compositions a small amount of a polytetrafluoroethylene resin or a fumed colloidal silica.
In addition, if the materials are compounded carefully, having due regard to the sensitive nature of the polyester, and attention is paid to careful drying of the resin and all other ingredients prior to compounding, there will be little or no degradation in physical properties, exemplified, for example, by a decrease in the apparent molecular weight, i.e., loss in intrinsic viscosity.